feat(completedDraw): adds "completedDraw" plugin events. Fixes #3289

[FGRibreau]

Copy/pasting our discussion with @etimberg 👍

there is no plugin call for after rendering fully finished (ie only after the last frame of animation). afterDraw will do after each frame
I would accept a PR adding it (could call it at the same place the onComplete animation callback is called)

So here it is @etimberg !

[etimberg]

Looks good to me.

[FGRibreau]

@zachpanz88 what are you thoughts on this? :)

[simonbrunel]

I'm not @zachpanz88 :p but want to add my thoughts: afterDraw already provides this functionality:

Chart.plugins.register({
    afterDraw: function(instance, easingDecimal) {
        if (easingDecimal !== 1) {
            // drawing during animation
            return;
        }

        // draw the last animation frame (completedDraw)
    }
});

A potential issue is that for the last animation frame, afterDraw and completedDraw will be called and might do exactly the same thing. My feeling, when it touches public APIs, is that adding duplicated ways to do the same thing could be confusing and for sure will be complicated to remove/change it later.

[FGRibreau]

@simonbrunel you are perfectly right, so maybe a simple documentation update that explains how to listen when the draw is complete is a better idea don't you think?

[simonbrunel]

That what I would suggest yes :)

We could replace // Easing is for animation by a more detailed note for the whole group of (before|after)(Datasets)?Draw methods (taking easing as parameter) that explain how to test easing to detect the last frame (i.e. render is complete).

[panzarino]

Looks good to me

[simonbrunel]

Just a note that we should maybe not add a new plugin API since afterDraw provides the same functionality. Instead, the documentation should be updated (see comments). @etimberg @zachpanz88 what do you think?

[panzarino]

My bad, I thought that you meant implement these changes and change the documentation. If afterDraw provides the exact same functionality there is no reason to add anything new.